The present invention relates to electronic circuits, such as to protection circuits against high currents in lighting converters. More particularly, but not exclusively, the invention relates to a protection circuit associated with a power device having an output terminal connected to an electric load and at least one control terminal receiving a predetermined driving current by a driving circuit.
In the following description reference will be made to an electric load represented by a halogen lamp or fluorescent lamp without being limiting thereto. In almost all the applications where lighting converter circuits are used, there are protection circuits that intervene when high currents flow in the converter for a time period longer than the time expected for the circuit start-up. The protection circuits are typically required to prevent some components from being destroyed or damaged.
The attached FIG. 1 schematically shows the structure of an AC/AC converter used for driving a halogen lamp as in the prior art. The structure of FIG. 1 is substantially an electronic transformer provided with a protection against short circuit on the load, since in the start-up phase the load current is much higher than the nominal current.
Differently from what happens with fluorescent lamps, the circuit 1 of FIG. 1 is powered by an external AC voltage source network, rectified at double half-wave. A diac 2 enables the converter circuit during each supply cycle. The circuit 1 comprises a power device 3 in each portion of a half-bridge structure including a pair of driving elements. More particularly, a high side driver component 4 and a low side driver component 5 are connected in series between a high supply voltage reference and ground GND.
The interconnection node X between the components 4 and 5 is connected to a halogen lamp 6. A first winding 7 is provided between the node X and the high side component 4, while a second winding 8 is provided between the node X and the second low side component 5.
The current Iload flowing in the lamp 6 is alternately switched, preferably at a frequency of 30 to 50 KHz, by the half-bridge branches. The high supply voltage is derived from the alternating current (AC) external supply through the diac 2. Several RC circuits are provided between the high supply voltage and the ground to obtain voltage values to be applied to the low side component 5 or to the high side component 4.
For these applications a circuit 9 shown in FIG. 2 is typically used, which serves to implement a gradual start-up, called a xe2x80x9csoft start-upxe2x80x9d. The circuit 9 has a first terminal connected to the voltage supply Valim, produced inside circuit 1, and a second terminal connected to the node X. This circuit 9 comprises a power bipolar transistor Q1 having conduction terminals, that is, collector and emitter terminals, coupled to the second terminal and to ground respectively. A sensing resistor Rsense is provided between the emitter and ground for measuring the current Ie flowing through the conduction terminals.
The base terminal B1 of the transistor Q1 is coupled to the first supply terminal by a diac D and a resistance R3. A second bipolar transistor Q2 has its conduction terminals, that is, its collector and emitter terminals, connected respectively to the transistor base B1 by the diac D and to ground. A capacitor Cd is connected in parallel between the driving terminal and conduction terminal of the transistor Q2.
A resistance R2 is provided between the base B2 of the second transistor Q2 and ground. An electrolytic capacitance C1 is included in a first circuit portion comprising the resistance R2 and an additional resistance R1 having a terminal is connected to the base B2. The capacitance C1 is also inserted in a second circuit portion comprising the resistor Rsense and a diode D1.
The electrolytic capacitance C1 is charged when the voltage drop Rsense*Ie is higher than the voltage sum Vbed1+VC1 and drives the transistor Q1. The time constant generated by the capacitance C1 and the resistance R1 has a high value and ensures that the transistor Q1 is kept in the on state for several half waves of the supply voltage waveform Valim.
The transistor Q1 performs the function of draining part of the current which would flow, though the resistance R3, on the capacitor Cd. This slows the corresponding charge and delays the start of the diac 2. This causes a shift of the instant in which, in the half wave of the supply voltage, the circuit 9 starts oscillating. Because of the gradual impedance variation inside the lamp, the currents become lower and lower and the transistor Q1 will have less base current available if the capacitance C1 is charged at a lower value.
Consequently, the diac 2 will be delayed by a lower time than the previous half wave. Therefore, the circuit 9 will keep on operating, but with a decreasing impact, until the current switched in the lower branch reaches the steady state value.
The circuit is disabled until the transistor Q1 has the required base current to be switched on. Therefore high time constants are needed so that, if a short circuit occurs, the circuit in the off state sustains several cycles of the supply voltage Valim, and this is so even for a few seconds.
Once the capacitance C1 charge is exhausted and the transistor Q1 is shut-off, the cycle starts oscillating with the highest current, just near the short circuit current, until the capacitance C1 reaches once again a useful signal for driving the transistor Q1. The capacitance C1 charge is strictly linked to the current value on the resistor Rsense. The circuit 10 is substantially a peak detector.
The half-bridge converters used to drive fluorescent lamps, for which the fluorescent tube replacement is provided, are provided with a ballast protection which intervenes when the tube is exhausted (EOL - End of Life condition). With reference to FIG. 3, this EOL tube state is represented schematically with an LC circuit having a low impedance and allowing the driving circuit to oscillate freely with high currents. In a normal start-up phase this condition is present until the tube is triggered. In this phase, the triggered tube is located in parallel with the capacitance C1, and the circuit 9, once the load impedance is changed, oscillates with low currents. In EOL conditions, through the conduction path formed by the components C2-R11-R21-D11 shown in FIG. 3, the electrolytic capacitor C3 is charged. This capacitor C3 potential enables the latch circuit 11 to be triggered, determining the low side component 5 switch-off and, thus ending the oscillation. The C3 charge time constant allows the EOL condition to be discriminated from the normal start-up condition.
In view of the foregoing background, it is therefore an object of the present invention to provide a circuit for protecting against high currents in lighting converters, and wherein the circuit has relatively simple structural and functional characteristics and allows adequate protection, particularly in the start-up phase, against power dissipation caused by high current oscillations.
The present invention detects the excessive increase of the power device temperature, due to the power dissipation in connection with high current oscillations, by using a thermal sensor integrated in the driving circuit. The sensor output is used in a thermal protection block which intervenes, according to the topology being used, to set the potential of appropriate and predetermined circuit nodes.
One embodiment of the invention is directed to an electronic protection circuit against high currents in lighting converters including at least one switching power device having an output terminal connected to an electric load and at least one control terminal receiving a predetermined driving current value by a driving circuit portion. The circuit also preferably includes an integrated temperature sensor detecting the temperature of the power device, and an output stage connected downstream of the sensor to switch off the driving circuit portion when a predetermined operation temperature is exceeded.